The correct option is A a popular belief
As mentioned in the sentence "Many legends tell of these cavemen-like soldiers ... folklore has it that they were physically transformed into the beasts ... .", this was a legend or folklore. 'Folklore' literally means 'traditional beliefs', 'mythology', it refers to a belief that has been passed from generation to generation, usually by word of mouth. Since they are historical stories, they are popular; hence the given statement "The animal skin had the power... " can be concluded as 'a popular belief'. So, option A is correct.
Option B mentions a customary practice, but the given statement is based on folklore, which may or may not be true. A customary practice refers to the manifestation of customs and traditions that are particular to religious and ethnic groups: like child marriage, inbreeding, etc. It is an actual practice carried out, hence the given statement does not mean the same as 'customary practice' and option B is incorrect.
Since it is already mentioned in the passage: 'folklore has it that...' and that folklore is a belief, it is not proven. A belief means 'acceptance of something, without proof'; the option is very much the opposite of what the sentence is: a belief. Hence option C is incorrect.
A divine act refers to any act that is related to God. However, wearing animal skin and changing into a beast has no relation to God. Also, it is nowhere mentioned in the passage, particularly in the line that states "the animal skin", that it is divine. Hence option D is incorrect.
(The line: 'unlikely event of a villager slaying a Badanga warrior, he was revered as a divine saviour...' is in relation to defeating a Badanga warrior, but that does not make the given statement 'the animal skin...into a beast' a divine act itself.)